1. Field of the Invention
The present invention relates to a novel catalyst and a process for preparing it. More particularly, it relates to a catalyst suitable for use in the production of acrylic acid by catalytic oxidation of acrolein in gas phase with molecular oxygen, and to a process for preparing the catalyst.
1. Prior Art
Catalysts for producing acrylic acid by catalytic oxidation of acrolein in gas phase are described in Japanese Patent Publication Nos. 41-1775 (1966) and 44-12129 (1969).
Japanese Patent Application Laying Open No. 47-8360 (1972) describes a catalyst comprising antimony, molybdenum, vanadium and tungsten as essential components and a trace amount of copper. Further, Japanese Patent Application Laying Open No. 48-96514 (1973) discloses a catalyst comprising molybdenum, vanadium, tungsten and tin as essential components and optionally antimony and/or copper.
Japanese Patent Application Laying Open Nos. 51-11709 (1976), 52-23589 (1977), 52-153889 (1977), 58-166939 (1983) and 3-218334 (1991) disclose a catalyst comprising copper, tin, antimony and chromium as optional components; a catalyst containing an antimony-nickel compound; a coated catalyst containing copper and antimony as optional components; a ring catalyst containing copper and antimony as optional components; and a high-yield, high-productivity catalyst, respectively. Although some of these catalysts have been manufactured on a commercial scale and used in the production of acrylic acid, the productivity thereof is not always satisfactory. There has recently been an increasing need for a catalyst with a higher productivity as the demand for acrylic acid is increased.
In recent years, an increased amount of acrolein supplied per unit volume of a catalyst ("high load condition") has been used to improve the productivity in the production of acrylic acid by catalytic oxidation of acrolein in gas phase. Since the oxidizing reaction of acrolein is exothermic, hot spots are created due to increased heat accompanied with the increased amount of acrolein. These hot spots tend to cause entrainment of, for example, molybdenum which is most frequently used as the element constituting a catalytically active component.
If a catalyst used in this reaction is prepared by compression, extrusion or coating, and if the mechanical strength of the catalyst is low, powders of broken or peeled catalytically active component may clog a reaction vessel upon being filled with the catalyst, resulting in abnormal pressure rise of the reaction vessel. Therefore, it is desirable for a compressed, extruded or coated catalyst to have excellent mechanical strength such as attrition resistance.